Hive Fleet Dagon was the first major Tyranid incursion into the Jericho Reach region of the Ultima Segmentum. The Shadow in the Warp cast by the Tyranid Hive Mind within this volatile region of space first heralded the arrival of this new and terrible Hive Fleet in 812.M41. Communication between parts of the Achilus Crusade and the re-conquered regions of the Jericho Reach was lost without warning. Particularly affected was the Orpheus Salient of the Crusade that was lost to communication and access with the rest of the Reach behind the Hive Fleet’s crawling shadow of Warp interference. In 814.M41, the Deathwatch warship Thunder’s Word entered the Freya System to investigate the silence of an Astropathic relay. It found no trace of the relay and indeed no trace of any life in the Freya System. Preparing to leave the system, the Thunder’s Word encountered two Tyranid drone ships. Its hull scored and pitted with acid and its magazines nearly exhausted, the Thunder’s Word arrived at Watch Fortress Erioch bringing news that the Tyranids had come to the Jericho Reach.

The Tyranid Hive Fleet that is assaulting the Jericho Reach is officially designated by the Imperium as Hive Fleet Dagon and has been identified as the greatest single extant threat to the Achilus Crusade underway in the region, and indeed to the lives of every living being in the Jericho Reach. The Space Marines of the Deathwatch remain fully engaged in facing this threat. Not much is known about the origins of Hive Fleet Dagon other than that it hails from the edge of the galactic rim, far to the galactic east like all of the Tyranid Hive Fleets that have been encountered to date. It is highly suspected by the Imperial savants of the Ordo Xenos that this particular Hive Fleet may be a splinter of the decimated Hive Fleet Behemoth, which was destroyed during the First Tyrannic War in 745.M41 by Imperial forces.